Liposomes are completely closed lipid bilayer membranes containing an entrapped aqueous volume. Liposomes may be unilamellar vesicles (possessing a single membrane bilayer) or multilameller vesicles (onion-like structures characterized by multiple membrane bilayers, each separated from the next by an aqueous layer). The bilayer is composed of two lipid monolayers having a hydrophobic “tail” region and a hydrophilic “head” region. The structure of the membrane bilayer is such that the hydrophobic (nonpolar) “tails” of the lipid monolayers orient toward the center of the bilayer while the hydrophilic “heads” orient towards the aqueous phase.
A primary use of liposomes is to serve as carriers for a variety of materials, such as, drugs, cosmetics, diagnostic reagents, bioactive compounds, and the like. Hydrophilic agents when entrapped in liposomes associate with the aqueous spaces in the liposome structure, primarily the internal central compartment and the spaces between lipid bilayers. Lipophilic drugs are typically associated with the lipid bilayer.
Liposome drug formulations can improve treatment efficiency, provide prolonged drug release and therapeutic activity, increase the therapeutic ratio and possibly reduce the overall amount of drugs needed for treating a given kind of ailment or disorder. For a review, see Liposomes as Drug Carriers by G. Gregoriadis, Wiley & Sons, New-York (1988).
Many methods exist for preparing liposomes and loading liposomes with therapeutic compounds. The simplest method of drug loading is by passive entrapment, wherein a dried lipid film is hydrated with an aqueous solution containing the water-soluble drug to form liposomes. Other passive entrapment methods involve a dehydration-rehydration method where preformed liposomes are added to an aqueous solution of the drug and the mixture is dehydrated either by lyophilization, evaporation, or by freeze-thaw processing method involving repeated freezing and thawing of multilamellar vesicles which improves the hydration and hence increases loading.
In general, however, passive entrapment yields a low efficiency of drug entrapment. A low drug-to-lipid ratio can be a significant disadvantage to achieving efficaceous therapy with a liposome formulation. For example, for treatment of tumor tissue, liposomes with a size of approximately 80-140 nm are required for extravasation into the tumor tissue. The small size imposes a limitation on the drug loading, especially when the drug is passively entrapped and/or when the drug has a limited solubility and/or the drug has a low affinity for the lipids. A low drug-to-lipid ratio requires administration of a large lipid dose to achieve the required drug dose.
The drug entrapment efficiency can be improved in part by using a high lipid concentration or by a specific combination of lipid components. For example, an amphiphatic amine, such as doxorubicin, may be encapsulated more efficiently into liposome membranes containing negative charge (Cancer Res. 42:4734-4739, (1982)). This surface electrostatic drug association with liposomes, while very stable in the test tube, leads to very rapid, almost immediate, release upon systemic administration.
Thus, loading of uncharged and non-lipophilic drugs, and in particular drugs having low solubility in aqueous phase at a sufficiently high drug-to-lipid ratio for efficaceous therapy remains a problematic issue.